Hitherto, ultra-precision tools to machine dies for optical elements, such as diffraction gratings, have been composed of natural or artificial single-crystal diamond (PTLs 1 and 2). However, in the case of tools composed of single-crystal diamond, the material has a property of cleaving along the (111) plane. Thus, tools are chipped or broken by stress during use, which is a problem. The amount of wear varies depending on crystal orientation (uneven wear). Only a specific plane is worn away in a short time as tools are used, and prolonged machining cannot be performed, which is a problem. The reason for this is that, for example, the amount of wear at the (111) plane is significantly different from that at the (100) plane.
As countermeasures against the cleavage cracking and uneven wear, it is conceivable that sintered diamond may be used. Such sintered diamond is obtained by sintering diamond grains with a metal binder, such as cobalt, so the metal binder is present among the diamond grains. However, the metal binder portions are softer than the diamond grains and thus worn away in a short time. The diamond grains usually have a diameter of 0.5 to 25 μm, which is larger than a radius of curvature of a cutting edge of 0.1 μm or less that is required for ultra-precision machining. As a result, uneven wear occurs in the same way as the single crystal, thereby failing to form a fine pattern in a large area by machining.
As polycrystalline diamond free from a metal binder, there is polycrystalline diamond obtained by a chemical vapor deposition method (CVD method). However, the polycrystalline diamond usually has a grain size of 1 μm or more and low intergranular bonding strength. Thus, the life is disadvantageously short.
Ultrananocrystalline diamond (UNCD) containing smaller grains can be synthesized by, for example, a pulsed vacuum arc plasma deposition method (NPL 1). UNCD contains a large amount of hydrogenated amorphous carbon and thus has low wear resistance, compared with normal diamond.
As tools including cutting edges composed of materials other than diamond, cubic boron nitride (cBN) tools having hardness next to diamond are known.
There are some types of cBN tools. In the case where rough machining is performed inexpensively, a tool obtained by sintering cBN with a binder is disclosed in, for example, PTL 3. In the case where the binder is present, however, precise machining cannot be performed. Examples in which binder-free sintered cBN compacts are used are disclosed in PTL 4 and NPL 2. However, in these cases, the grain size of cBN was as large as 50 to 500 nm. Thus, it is difficult to perform ultra-precision machining using the binder-free cBN sintered bodies at a level required for optical components.
In recent years, progress has been made in sintering techniques. As disclosed in PTL 5, sintered cBN compacts having a grain size of less than 50 nm have been obtained. Attempts have been made to use them as tools. Here, an example is disclosed in which an R tool having a radius of curvature of 500 μm is produced by polishing and stainless steel is cut. However, this is a simple prototype of a tool of the order of micrometers. For example, no attempt has been made to form an ultrafine diamond grooving tool for nanoscale machining required to produce optical elements.
The inventors have attempted to produce a grooving tool and a V-shaped tool from similar fine-grain cBN. However, the fine-grain cBN had higher hardness than a normal cBN material; hence, microchipping occurred during polishing, failing to achieve sufficient accuracy as precision tools.
Such cutting tools composed of diamonds and cBN described above were machined and formed by polishing. Examples in which diamond tools are machined with a focused ion beam (FIB) in order to increase precision of ultra-precision cutting tools are disclosed in PTLs 6 and 7. However, from results of studies by the inventors, for example, in the case where a method described in PTL 6 was applied to a V-shaped tool, machining was performed by the irradiation of an ion beam only from the side of a rake face. Thus, a ridge between the rake face and a flank face was rounded to increase the radius of curvature. Therefore, the tool was not used for ultra-precision machining. PTL 7 discloses machining of either front portion or rear portion of a cutting edge. Similarly, for example, in the case where the technique is applied to a V-shaped tool, in the former, the radius of curvature of a ridge between a rake face and a flank face was increased in the same way as the method described in PTL 6. In the latter, the radius of curvature of a ridge between two flank faces was increased. In any case, the technique was not sufficient for ultra-precision machining.
PTL 8 discloses a method for producing a diamond tool, a cubic boron nitride tool, or the like by allowing a focused ion beam (FIB) to enter a flank face and then to emanate from a rake face. However, the results of studies by the inventors demonstrated that in this method, in particular, the radius of curvature of a ridge between two flank faces was increased and that the ridge was rapidly rounded as wear proceeds by cutting; hence, the method was not sufficient for ultra-precision machining over a longer distance.